1. Field of the Invention
The present invention is directed to a method for generating an image by magnetic resonance, and in particular to such a method employing a number of reception antennas, for picking up magnetic resonance signals, the respective antennas having different sensitivity profiles.
2. Description of the Prior Art
In the measurement sequences for magnetic resonance imaging that have been standard, and employed with a given size and resolution of the imaging, the time required for generating a magnetic resonance image is defined by the intensity of the gradient magnetic field used for the topical resolution. The gradient coils with which the gradient magnetic field is generated are becoming increasingly powerful and the measurements are becoming increasingly faster as a result. However, a physiologically prescribed limit human tissue (stimulation limit) that cannot be transgressed exists because of the magnetic fields that are rapidly switched in such sequences and because of the electrical voltages that are thereby induced in the tissue of the patient.
Recently, methods have been developed that are referred to as coil sensitivity and coding methods, or partial parallel acquisition (PPA). These methods use the sensitivity profiles of the individual antennas of an antenna array in order to reduce the phase-coding steps needed for the topical resolution and to thus shorten the measurement time.
Thus, the article by Hutchinson and Raff, xe2x80x9cFast MRI Data Acquisition Using Multiple Detectorsxe2x80x9d, in Magnetic Resonance in Medicine, Vol. 6, pp. 87-91 (1988), describes a method wherein only one phase-coding step is required for producing an image. An antenna array having a number of independent individual antennas and radio-frequency channels is employed, the number corresponding exactly to the number of phase-coding steps given conventional, sequential phase-coding with phase-coding gradient fields. This method is difficult to employ because of the high number of required reception channels.
The article by James R. Kelton, Richard L. Magin, Steven M. Wright, xe2x80x9cAn Algorithm For Rapid Image Acquisition Using Multiple Receiver Coilsxe2x80x9d, Proceedings of the SMRM 8th Annual Meeting, Amsterdam, 1989, p. 1172, describes a measuring method wherein the idea of Hutchinson and Raff was expanded. The number of individual antennas in the antenna array described therein amounts to a power of two. The measuring time is reduced corresponding to this number. The number of independent radio-frequency reception channels can be selected significantly lower then the number of phase-coding steps otherwise required for the image determination.
J. B. Ra and C. Y. Rim, in the article xe2x80x9cFast Imaging Method Using Multiple Receiver Coils with Sub-Encoding Data Setxe2x80x9d, which appeared in Proceedings of the SMRM 10th Annual Meeting, San Francisco, 1991, p. 1240, have described a method wherein, despite an under-sampling in the phase-coding direction, an unambiguous, convolution-free imaging of a region to be imaged (field of view) can be achieved. To this end, the reception signals of a number of independent reception antennas are reconstructed into intermediate images using a Fourier transformation, but these intermediate images are still ambiguous. Using the sensitivity profiles of the antennas employed, these intermediate images are processed to form a convolution-free ultimate image.
As reported in the article by J. B. Ra, C. Y. Rim, xe2x80x9cFast Imaging Using Sub-Encoding Data Sets from Multiple Detectorsxe2x80x9d, in Magnetic Resonance in Medicine, Vol. 30, pp. 142-145, 1993, the method outlined above was tested at a phantom with a four-channel system. A speed-up of the measuring time with a factor of 4 was thereby achieved. A method also is described in this article with which the speed-up factor can be selected lower then the number of independent reception antennas.
A development of the fast imaging method described by Kelton et al and Ra/Rim is disclosed in PCT Application WO 99/54746. The inverse sensitivity matrices required in the processing of the intermediate images are replaced in this version by generalized inverse sensitivity matrices. For defining the sensitivity profiles required for the reconstruction of the ultimate image, a reference measurement having the same or a lower resolution as in the actual image production is implemented before the actual registration. To that end, the magnetic resonance signals are measured with the individual antennas in the antennas array as well as with the whole-body antenna permanently installed in the magnetic resonance apparatus. The sensitivity profile of the whole-body antenna is constant enough in order to be able to be used as a reference. The complex images (in the mathematical sense) of the individual antennas obtained after the Fourier transformation and the reference image of the whole body antenna are placed into relationship with one another, and the complex sensitivity profiles (in the mathematical sense) of the individual antennas are obtained. These are then employed for the reconstruction in the following, actual measurement. A disadvantage of this version is that the required measuring time is lengthened by the xe2x80x9cpre-scanxe2x80x9d. It generally applies in the measurement of magnetic resonance images that the signal-to-noise ratio is proportional to the square root of the measurement time. Since the pre-scan, however, is employed only in order to determine the sensitivity profiles of the individual antennas in the antenna array, the signal-to-noise ratio is not improved despite a lengthened measurement time. The relationship of the signal-to-noise ratio to the square root of the measurement time is poorer in this version than in conventional methods when the measurement time required for the pre-scan is also taken into consideration.
U.S. Pat. No. 5,910,728 discloses another method with which the measurement time can be reduced by omitting phase-coding steps. An antenna array having independent individual antennas is also employed therein. The reconstruction of the missing phase-coding steps ensues, however, in the spatial frequency domain (k-space) and not in the image space as in the above methods. Due to the specific type of reconstruction of the missing k-space rows, this method is also called SMASH (simultaneous acquisition of special harmonics). It is assumed, however, that the sensitivity profiles of the individual antennas do not vary greatly in the frequency coding direction. Another pre-requisite for a convolution-free reconstruction of a magnetic resonance image is an exact knowledge of the sensitivity profiles of the individual antennas employed. Since the sensitivity profiles are also patient-dependent, they must usually be measured with each patient in the examination position.
In the method described in the article by P. M. Jakob, M. A. Griswold, R. R. Edelman, D. K. Sodickson, xe2x80x9cAUTO SMASH: A self-calibrating technique for SMASH imagingxe2x80x9d, in 1998 in Magnetic Resonance Materials in Physics, Biology and Medicine, Vol. 7, pages 42-54, a calibration step with a corresponding phase-coding is implemented in addition to the reduced SMASH phase-coding steps. The sensitivity profiles of the individual antennas are determined therefrom during the measurement, and the correlation between the under-sampled SMASH signals and the additional calibration signals is analyzed.
An object of the present invention is to provide a method for fast image generation by means of magnetic resonance, whereby the sensitivity profiles of the individual antennas in the antenna array are determined in a simple way.
This object is achieved in accordance with the invention in a method for image generation by magnetic resonance, wherein a number of independent reception antennas having sensitivity profiles differing from one another are employed, and wherein in radio-frequency excitation pulses and gradient pulses are emitted into an imaging volume for generating location-coded magnetic resonance signals, the magnetic resonance signals are received with the reception antennas, with a k-space data set having middle and outer k-space rows being formed from the reception signals of each reception antenna, the middle k-space rows being more densely arranged than the outer k-space rows in the k-space data set, an intermediate image is reconstructed from each k-space data set, the sensitivity profiles are determined from the middle k-space data rows, and the intermediate images are combined dependent on the sensitivity profiles to form an overall image.
In this method, the measurement steps for determining the sensitivity profiles of the individual antennas are integrated into the actual measurement. This ensues by the k-space in the low spatial frequency range being completely occupied in phase-coding direction using the phase-coding gradient fields. Outside this region, i.e. in the region of high spatial frequencies, the phase-coding steps are reduced according to the methods proposed by Ra/Rim or Kelton et al in order to reduce the measurement time.
Individual images having a rough (coarse) resolution are then reconstructed from the magnetic resonance signals in the low spatial frequency range, these individual images being allocated to the corresponding individual antennas. Since the sensitivity profiles of the individual antennas are themselves mainly composed of low spatial frequency components, the information needed for determining the sensitivity profiles is thus completely identified. No additional information beyond the sensitivity profiles of the individual antennas is required.
The entire measurement time is lengthened somewhat as a result. The additional phase-coded magnetic resonance signals in the middle region of the k-space, however, can be employed for enhancing the signal-to-noise ratio, and thus for improving the imaging quality for the reconstruction of the individual images as well, and thus of the overall image. It is also a particular advantage that the sensitivity profiles are derived from the same measurement as the signals acquired for the actual image generation; this method therefore functions for determining the sensitivity profiles even given sequences with more pronounced topical distortion. One example of this is the EPI sequence (EPI is an abbreviation for echo planar imaging). It is known that image artifacts referred to as blurring artifacts or distortions occur given fast EPI sequences. Since the reference images exhibit the same distortions, the reconstruction also functions given more pronounced topical distortion.
It should be noted out that filters are required in the case of a method with a pre-scan in order to obtain noise-free coil profiles. These filters must be adapted to the size of the field of view and to the coil size, which requires a certain outlay for the implementation. In contrast, the coil profiles need not be filtered in the inventive method. The employment of signals from only the middle region of the k-space corresponds to a low-pass filtering.